1. Field of the Invention
The present invention relates to a button holer sewing machine capable of forming a button hole seam in a workpiece cloth set on a feed bar by horizontally moving the feed bar and vertically moving a needle bar up and down with an electric motor.
2. Description of the Related Art
There has been known a button holer sewing machine including a lower knffe and an upper hammer. The lower knife is positioned below the upper surface of the feed bar and retracted a predetermined distance behind a needle bar of the sewing machine. The hammer is provided above the lower knife so as to be pivotally movable toward the knife. When a workpiece cloth is set on the feed bar of the sewing machine and the start switch of the sewing machine is pressed, then the hammer pivots against the cloth so that the knife cuts a hole in the workpiece cloth. Next, the feed bar is moved frontwardly a distance determined by the above-described predetermined distance minus the sewing length, that is, the length of the button hole to be sewn. At this point, the sewing machine is started up. Next, the feed bar is moved in synchronization with vertical movements of the needle bar. The needle bar is pivoted in predetermined amounts. That is, when the needle bar is descending, it is alternately brought to an outer stitch position and an inner stitch position so that a desired hole seam is sewn.
Button holer sewing machines are capable of forming button hole seams using two methods: the first type is called the cut-first method, wherein sewing is performed after the hole is opened, and the second type is called the cut-last method, wherein the hole is opened after sewing is performed. A user selects the type of sewing method most appropriate to the user's need. When the cut-first method is used, seam data is prepared so as to stitch slightly inner side portions of the hole. On the other hand, when the cut-last method is used, seam data is prepared so as to stitch slightly outer side portions of the hole.
With the conventional button holer sewing machines, when the stitch width is to be changed, the pivotal amount of the needle bar between the inner stitch position and the outer stitch position is changed. It takes time even for a skilled worker to change the pivotal amount of the needle bar. Therefore, changing the stitch width lowers working efficiency.
Further, with conventional button holer sewing machines having a knife and hammer configuration, the lower knife must be frequently replaced to change the hole shape in accordance with the product to be produced. However, when the lower knife is attached with poor precision, holes are not opened at the ideal position with respect to the seam. The resultant button holes have a poor appearance. When the cut-last method is used, in the worst situation, the seams can be cut by the knife, thereby ruining the finished product. For this reason, the attachment of the lower knife must be performed by a skilled worker and with great care, which is troublesome.